types of particles, Coarse Particles and Ultrafine Particles. Human activities, such as the burning of fossil fuels in vehicles, stubble burning, power plants, road dust, wet cooling towers in cooling systems and various industrial processes, generate significant amounts of particulates. Coarse particles (PM10) can penetrate the deepest part of the lungs such as the bronchioles or alveoli. Ultrafine Particles (PM2.5), tend to penetrate into the gas exchange regions of the lung (alveolus), and very small particles (ultrafine particulate matter, PM10) may pass through the lungs to affect other organs. Present day air pollution In April–May 2016, ozone and suspended matter pollution in Mexico City reached levels that were detrimental to health, though the criterion to signal a pollution alert was lower in 2016 than in the 1980s. The city's population continues to grow, to spread out, which lengthens automobile trips, and the number of autos in the city increases yearly. The air-quality monitor system (IMECA) is the tool used by the Mexican Authorities to measure air quality. A total of 8 system monitors, located all around Mexico City measure the levels for the 6 main pollutants that are found in the atmosphere of the city. The measured pollutants are: O3, PM10, PM2.5, CO, NO2, SO2. In addition, the stations have a meteorological tower, equipped with sensors that measure the ambient temperature, wind speed and direction, precipitation, solar radiation, relative humidity and barometric pressure. Indicators for each pollutant allow to assess the state of air quality, with respect to specific pollution events. Indicators of the maximum hourly concentrations and daily mobile averages were used to evaluate the behavior of the concentrations presented by the criteria pollutants and their reference to health protection standards. IMECA is the index used to display the level of pollution and the level of risk that represents to the human health in the greater Mexico City as well as the time of the measurements or the actions recommended for protection. To report the quality of the air, the IMECA uses 5 index categories: Good — when the index is between 0 and 50 imecas. The quality of air is considered satisfactory and the air pollution poses minimal or no health risks. Fair — when the index is between 51 and 100 imecas. The quality of air is acceptable, however some contaminants could have moderate health effects for a small fraction of the population that has high sensitivity